The present invention relates to a method of manufacturing a tie rod bar for a steering linkage of a motor vehicle, and more particularly to a method of manufacturing a tie rod bar with a hollow structure using as a material a steel pipe cut into a given length.
As a tie rod bar for use in a rack and pinion type steering linkage of a motor vehicle, there has hitherto been known a tie rod bar, for example, as shown in FIG. 10. The prior tie rod bar 1 has a sphere section 2 at its one end portion and a neck section 3 adjacent to the sphere section 2 and further a shaft section 4 adjacent to the neck section 3, with a boot groove 5 being made in the shaft section 4. In addition, the prior tie rod bar 1 has a screw section 6 at its other end portion, which is screwed in a screw hole 8B made in an extending section 8A of a socket 8 of a ball joint 7. Further, a nut 9 tightens the extending section 8A to prevent the screw section 6 from getting loose. The socket 8 has a housing hole 8C in which a sphere section 10A of a ball stud is accommodated through a bearing sheet to be slidable. Further, a dust cover 11 is fitted between the socket 8 and the ball stud 10, and a bottom edge portion of the socket 8 is caulked with respect to an end cover 12. The sphere section 2 of the tie rod bar 1 is rotatably slidably encased within a housing hole 13A of a socket 13 through a bearing sheet 14, while a shaft section 15 constituting a screw section 15A is formed to protrude from the socket 13. A boot 16 is fitted between the socket 13 and the boot groove 5 of the shaft section 4.
The tie rod bar 1 is produced by forming a circular bar material through a cold forging or hammering and then by performing a mechanical process (cutting process). In addition, recently, for the reduction in weight, the tie rod bar has been manufactured by a swaging process using as a material a steel pipe cut into a given length.
However, in the case of employing the prior method of manufacturing the tie rod bar with a hollow structure using the steel pipe material in the swaging process, the wall thickness varies by location so that the strength of a thin portion becomes unsatisfactory. Further, since the steel pipe material receives a load from both sides in the swaging process, wrinkles can arise on its inner surface to cause the occurrence of cracks. Moreover, the swaging process itself can not impart a sufficient hardness to the material, with the result that the tie rod bar can not have a sufficient strength. Further, the strength varies among the tie rod bars so that the product quality becomes unstable.
Furthermore, although in the prior tie rod bar its boot groove is formed by the swaging process or a rolling process (roll forging), the swaging process takes time to form the boot groove, thus deteriorating the productivity. Further, the roll forging can not form the boot groove in a single operation because the boot groove has a straight configuration, that is, it requires 4 to 5 operations using 4 to 5 kinds of roll dies. In addition, when the boot is mounted in the boot groove, the contact surface pressure is low because the boot groove has the straight configuration so the airtightness becomes unsatisfactory. Moreover, both the corner edge portions of the boot groove tend to become broken, therefore making it difficult to arrange their shapes. If the shape is arranged by force through the rolling, the wall thickness is reduced so that it becomes difficult to ensure its strength.